Semiconductor devices such as Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) are commonly used as power devices in applications, such as automotive electronics, power supplies, telecommunications, which applications require devices to operate at currents in the range of tenths up to hundreds of amperes (A).
Conventionally, by applying a voltage to the gate electrode of a MOSFET device, the device is turned on and a channel will be formed connecting the source and the drain regions allowing a current to flow. A lightly doped drift region is formed between the drain region and the channel. The drift region is required to be lightly doped in order to lower the maximum electric field that develops across the drift region and to thus, ensure a high breakdown voltage. Once the MOSFET device is turned on, the relation between the current and the voltage is nearly linear which means that the device behaves like a resistance. The resistance is referred to as the on-state resistance Rdson.
Typically, MOSFET devices with low on-state resistance Rdson are preferred as they have higher current capability. It is well known that the on-state resistance Rdson may be decreased by increasing the packing density of a MOSFET device i.e. the number of base cells per cm2. For example, a hexagonal MOSFET (HEXFET) device comprises a plurality of cells, each cell having a source region and a hexagonal polysilicon gate, and has a high packing density e.g. 105 hexagonal cells per cm2. Due to the large number of cells and the aspect ratio which may be defined as the ratio between the length of the hexagonal perimeter of the source region and the area of the unit cell, the on-state resistance of a HEXFET device can be made very low. Usually, the smaller the size of the cells, the higher is the packing density and thus, the smaller the on-state resistance. Therefore, many improvements to MOSFET devices are aimed at reducing the size of the cells.
However, it is well known that the breakdown voltage of MOSFET devices increases as the on-state resistance Rdson of the devices increases. Thus, there is a trade-off between reducing Rdson and having a high enough break down voltage BVdss.
In an attempt to reduce the on-state resistance Rdson of a MOSFET device whilst not impacting significantly the breakdown voltage of the device, it has been proposed to introduce multilayer structures in the drift region of the device. An article entitled ‘A Novel High-Voltage Sustaining Structure with Buried Oppositely Doped Regions’ by Xing Bi Chen, Xin Wang and Johnny K. O. Sin, in IEEE Transactions on Electron Devices, Vol. 47. No. 6, June 2000 describes a structure with buried floating regions in the drift region of the MOSFET device connected together at an edge termination. In the case of p-type buried floating regions in a n-type drift region, due to the negative charges in the depleted p-type buried floating regions, a large part of the flux induced by the positive charges of the depleted n-drift region are terminated on the buried floating regions so that the field intensity is not allowed to accumulate throughout the entire thickness of the drift region. This means that a larger doping density can be used in the drift region without producing a high peak field. Since a larger doping density in the drift region can be used, the on-state resistance Rdson is reduced. Thus, by using buried floating regions, the resistivity and/or thickness of the drift region can be made smaller than that of a conventional MOSFET device with the same breakdown voltage and therefore, the on-state resistance Rdson can be reduced.
When the MOSFET device is turned on, the on-state resistance is momentarily high due to the drift region and the buried floating regions being fully depleted, and only a small current flows in the channel. In order to turn the device on fully (ie. unblock the device), the majority carriers in the buried floating regions and the drift region have to be recovered. A MOSFET is an unipolar or a majority carrier device and thus, when the channel is opened, electrons can be easily recovered for the n-type regions (e.g. the drift region) from the source. However, it is more difficult to recover holes for the p-type buried floating regions as a unipolar device cannot provide any holes to these regions.
Thus, there is a delay between turning on a device and the device being fully on which delay depends on the time required to recover the majority charge carriers in the depleted regions. Ideally the aim when designing devices is to arrange for the delay to be zero or as close to zero as possible.
In the arrangement described in the article by Chen et al, as the p-type buried floating regions are connected together at a p-type edge termination which is connected to the source through a p-type body region, when the MOSFET device is turned on after being turned off, a path is established which provides holes to the p-type buried floating regions which are depleted during the reverse drain voltage. This helps in the process of recovering the majority carriers in the buried floating regions which enables the MOSFET device to be turned on fully. In view of the connection to the edge termination, the p-type buried floating layers are not electrically isolated from the source and so are not properly ‘floating’.
Since the p-type edge termination must be carefully designed to avoid any leakage issues and extra steps are required to form the edge termination, the arrangement disclosed in this article is costly and complex to manufacture. Furthermore, such an arrangement is not compatible with typical planar processes used in semiconductor device manufacturing processes.
An article entitled ‘A Novel Low On-Resistance Schottky-Barrier Diode with p-Buried Floating Layer Structure’ by Wataru Saito, Ichiro Omura, Ken'ichi Tokano, Tsuneo Ogura and Hiromichi Ohashi in IEEE Transactions on Electron Devices, Vol. 51, No. 5, May 2004 discloses a Schottky-Barrier Diode (SBD) having p-type buried floating layers in the drift region which enhance the development of depletion in the drift region. In the arrangement disclosed in this article, the buried floating layers are isolated from the p regions at the surface of the SBD so that when the MOSFET device is turned off and a reverse voltage is applied between the drain and source regions, the p-type buried floating layers and the drift region are fully depleted and there are no longer any free charge carriers in the drift region before breakdown is reached, which results in a high breakdown voltage. The result is the doping concentration of the drift region can be increased and so the on-state resistance of the SBD can be reduced.
When the device is turned on, the on-state resistance is momentarily high due to the drift region and the buried floating layers being fully depleted, and only a small current flows in the channel. In order to turn the device on fully (ie. unblock the device), the majority carriers in the buried floating layers and the drift region have to be recovered. This article identifies the problem of recovering holes for the p-type buried floating layers and suggests a mechanism by which the majority p charge carriers or holes can be recovered in the p-type buried floating layers by the parasitic action of a bipolar device set up by the p-type guard rings and p-type grids at the surface of the SBD. However, the analytical solution disclosed in this article relies on a parasitic bipolar action whose impact on device behaviour is difficult to control. Thus, such a parasitic solution is hard to apply to semiconductor devices manufactured for applications which require greater control over the device behaviour.
There is therefore a need for an improved semiconductor device arrangement.